Muscle cells use a variety of energy sources to power their contractions. For quick energy, the cells utilize their stores of ATP (adenosine triphosphate) and creatine phosphate, which is another phosphate-containing compound. These stored molecules are usually depleted within the first twenty seconds of activity. The cells then switch to other sources, most notably glycogen, a carbohydrate that is made of glucose molecules strung together in long-branching chains.